When data is transmitted between chips through an internal transmission path, or when data is transmitted between a transmitter and a receiver through an external transmission path, signal distortion may occur if the bandwidth of the transmission path is insufficient.
For instance, a baseband signal of several hundred MHz can be expressed as a sum of pure sine waves in a frequency domain. At this time, high-frequency components may be attenuated, so that the signal distortion may occur in a period where a great number of high-frequency components exist.
For this reason, the receiver does not receive the signal having sufficient size and shape, causing difficulty in signal analysis.
In order to solve the above problem, a pre-emphasis scheme is adopted. The pre-emphasis scheme is an equalization technique realized in a transmitter terminal.
According to the pre-emphasis scheme, a signal period having a great number of high-frequency components is amplified more than other signal periods by taking the attenuation of the high-frequency components into consideration before a signal is transmitted. Thus, the bandwidth can be enlarged and the transmission speed can be increased if the pre-emphasis scheme is adopted.
FIG. 1 is a graph showing an example of signal processing by using the pre-emphasis scheme, in which the x-axis represents time and the y-axis represents current.
Referring to FIG. 1(a), signal distortion occurs when a signal change is generated (see dashed lines). Thus, as shown in FIG. 1(b), the signal periods subject to the signal change are amplified at a predetermined rate according to the pre-emphasis scheme before the signal is transmitted.
However, if the signal periods are amplified through the pre-emphasis scheme by controlling a current source, the following problems may occur. First, because a current source control circuit is complicated, the driving of the current source control circuit is difficult and a circuit area is enlarged. Second, the transmission time must be synchronized between the transmitter and the receiver according to the control of the current source. Third, when a DC current source operates at a high speed, noise may be generated.